Mechanisms of calcium and sodium fluxes in anoxic myelinated central nervous system axons.

Electron probe X-ray microanalysis was used to measure water content and concentrations of elements (i.e. Na, K, Cl and Ca) in selected morphological compartments of rat optic nerve myelinated axons. Transaxolemmal movements of Na+ and Ca2+ were modified experimentally and corresponding effects on axon element and water compositions were determined under control conditions and following in vitro anoxic challenge. Also characterized were effects of modified ion transport on axon responses to postanoxia reoxygenation. Blockade of Na+ entry by tetrodotoxin (1 microM) or zero Na+/Li(+)-substituted perfusion reduced anoxic increases in axonal Na and Ca concentrations. Incubation with zero-Ca2+/EGTA perfusate prevented axoplasmic and mitochondrial Ca accumulation during anoxia but did not affect Na increases or K losses in these compartments. Inhibition of Na(+)-Ca2+ exchange with bepridil (30 microM) selectively prevented increases in intra-axonal Ca, whereas neither nifedipine (5 microM) nor nimodipine (5 microM) influenced the effects of anoxia on axonal Na, K or Ca. X-ray microanalysis also showed that prevention of Na and Ca influx during anoxia obtunded severe elemental deregulation normally associated with reoxygenation. Results of the present study suggest that during anoxia, Na+ enters axons mainly through voltage-gated Na+ channels and that subsequent increases in axoplasmic Na+ are functionally coupled to extra-axonal Ca2+ import. Na+i-dependent, Ca2+o entry is consistent with reverse operation of the axolemmal Na(+)-Ca2+ exchanger and we suggest this route represents a primary mechanism of Ca2+ influx. Our findings also implicate a minor route of Ca2+ entry directly through Na+ channels.